Model deviance associated with including, in a polychotomous regression model of profusion categories, each GEI formed from the combination of powers of time since exposure (a) and concentration (b). Model deviance associated with including, in a polychotomous regression model of profusion categories, each GEI formed from the combination of powers of time since exposure (a) and concentration (b). Age groups 50–74, post-1964 exposure. D Buchanan et al. Occup Environ Med 2003;60:159-164 ©2003 by BMJ Publishing Group Ltd